A system is known for forming a hollow fan blade that implements a technique of hot superplastic inflation on a blank made up of a plurality of welded-together “sheets” for the purpose of deploying the sheets inside a die by inflating them at high temperature, typically about 900° C. for titanium sheets. At that temperature, the metal can deform plastically and take up the shape of the inside wall of the die when said blank is placed inside said die and is inflated by means of a gas under pressure. The blade obtained in that way is particularly strong and light in weight. Such a method is used in particular for making large-chord blades for the fan of a turbojet.
More precisely, the above-mentioned prior art method comprises the following operations in particular:                fabricating component parts for the blade, i.e. two outer sheets and a central sheet;        depositing a diffusion barrier in a predefined pattern between each outer sheet and the central sheet;        assembling together the three sheets and applying diffusion-welding thereto so as to bond together all their areas in contact on which the diffusion barrier has not been deposited;        shaping the resulting blank to take up an aerodynamic profile;        inflating the blank by applying gas pressure to its inside at a temperature that makes superplastic inflation possible; and        proceeding with final machining, in particular trimming the inflated blank so as to obtain a hollow blade.        
The operation of superplastic inflation is a step in the above-summarized process that is particularly difficult and expensive. At present, each blade is formed by placing the blank in a hollow die that is very thick in order to withstand the pressure, until the two outer sheets come to fit against the inside surface of the hollow die. During this superplastic forming operation, the central sheet becomes deployed in such a manner as to constitute internal stiffening reinforcement. The die which is raised to high temperature must be capable of withstanding the high pressure that is applied thereto for a period of time that is quite long. On average about 2 hours (h) are needed to shape such a blade. The weight and the expense of the tooling are thus considerable, since the tooling must be dimensioned to accommodate the inflation forces and also the twisting forces that result from the special shape of the blank that has already been given an aerodynamic profile. In addition, the thermal inertia associated with that type of thick-walled die is considerable and contributes to increasing the above-mentioned cycle duration. Temperature gradients are also increased, and that leads to a reduction in the lifetime of the tooling.
In another field, U.S. Pat. No. 4,951,491 describes a technique for superplastic forming of a single sheet placed in tooling that is itself placed inside an autoclave. The two portions of the tooling are pressed on either side of the periphery of the sheet that is to be deformed.
In another possible implementation, a pressure difference is established inside the die on either side of the sheet that is to be deformed. That requires pressure to be exerted on the tooling that is greater than the inflation pressure in order to ensure that the tooling is closed. Mechanical stresses thus remain large even though the closure pressure is isostatic. It is difficult to establish the required pressure difference. That is why, in a second implementation, the tooling is closed by a mechanical system based on bolts. That second technique presents two drawbacks. Bolting is performed at ambient temperature prior to putting the autoclave into operation. At ambient temperature it is practically impossible to deform the blank in order to achieve good sealing between the blank and the two elements of the die. It is therefore very difficult to seal the die. It is doubtful whether such a bolt system can be reused after a single high temperature inflation cycle. It is therefore necessary to provide a bolted fastener system for each part that is made in that way. The invention serves to solve the above-mentioned problems.